Accessory films based on micro-louvers have already been used on mobile display screens to achieve optical data protection in a so-called privacy mode. However, such films are not capable of being switched between modes but have to be applied and removed manually. Also, they have to be carried separately from the display screen when not in use. Another substantial disadvantage is the light loss accompanying the use of such louver films
U.S. Pat. No. 6,765,550 describes such privacy protection provided by a filter of micro-louvers. Here, the biggest disadvantages are the need to mechanically attach or remove the filter, and the light loss in the protected mode.
U.S. Pat. No. 5,993,940 describes the use of a film the surface of which is provided with small, regularly arranged prismatic strips to create a privacy mode. The development and fabrication of this film are quite complicated.
In WO 2012/033583, switching between free and restricted viewing is effected by the triggering of liquid crystals arranged between so-called “chromonic” layers. This involves a light loss, and implementation is rather complicated, too.
US 2009/0067156 discloses a great number of ideas to configure an illuminating system and a display device. In particular, the version illustrated there in FIGS. 3A and 3B uses two backlights consisting of wedge-shaped light guides, and an LCD panel, where the posterior backlight 40 is intended to positively create a wide illuminating angle, and the anterior backlight 38 is intended to positively create a narrow illuminating angle. It remains unclear, however, in what manner the backlight 38 is meant to create a narrow illuminating angle without converting the light having a wide illuminating angle, originating from backlight 40, essentially into light having a narrow illuminating angle when it passes backlight 38.
Regarding the configuration shown in FIG. 5 of US 2009/0067156, it should be noted that the two light guides 46 and 48 each produce “narrow light”, i.e. light with a narrow illuminating angle. Conversion of the light in light guide 48 into “wide light”, i.e. light with a wide illuminating angle, is only achieved by means of a partial mirror 50, which has to be provided with prism structures in a complex process. This conversion extremely diminishes the light intensity, because the light that at first exits in a narrow illuminating angle (the only light available) is then spread out into a wide illuminating angle, as a rule into the semispace. As a result, the brightness (related to the luminance) will be reduced by a factor of 5 or higher, depending on the parameters. Thus, this configuration is of little practical relevance.
In the embodiment according to FIG. 7 of US 2009/0067156, a phosphorus layer that converts UV light into visible light is an absolute must. This is rather laborious to do; and given the aim to get sufficient light from the backlight to illuminate an LCD so that it can be read, one needs very high UV intensities. Therefore, this configuration is expensive and complicated; shielding off the UV radiation alone makes it impracticable.
US 2012/0235891 describes a highly complex backlight in a screen. According to FIGS. 1 and 15, this design employs not only several light guides but also other complex optical elements such as microlens elements 40 and prism structures, which convert the light coming from the posterior illumination on the way to the anterior illumination. This is expensive and complicated to implement, and it involves a light loss. According to the version shown in FIG. 17 in US 2012/0235891, both light sources 4R and 18 produce light having a narrow illuminating angle, with the light radiated by the posterior light source 18 first being laboriously converted into light with a large illuminating angle. This complex conversion greatly diminishes brightness, as noted already above.
According to JP 2007-155783, special optical surfaces 19 that are difficult to compute and to manufacture are used to deflect light into varied narrow or broad regions depending on the light incidence angle. These structures are similar to Fresnel lenses. Furthermore, there exist inactive edges, which deflect light into unwanted directions. Thus, it remains uncertain whether really useful light distributions can be achieved.
For achieving restricted vision as taught by GB 2428128 A, additional light sources, which are arranged at a distinct distance from the screen, and illuminate a hologram attached to the screen, are used to overlay the lateral view with special wavelengths. The disadvantages here are the necessary spacing of the light sources from the screen, and the complexity of making suitable holograms.
US 2013/0308185 describes a special light guide provided with steps, which radiates light at a large area into various directions, depending on the direction in which it is illuminated from an edge. In interaction with a transmissive imager, e.g., an LC display, a screen that is switchable between a free and a restricted viewing mode can be produced. Here, a disadvantage, among others, is that the restricted view effect can be created either for left/right or for top/bottom only, bot not for left/right/top/bottom simultaneously as desirable for certain payment actions. In addition, some residual light is visible from blocked viewing angles even in the restricted viewing mode.
Finally, DE 10 2014 003 298 A1 describes a method and arrangement for the optional restriction of the recognizability of images. For this, a special optical element is needed, which is transparent to at last 70% of the light emanating from the screen, and which deflects light incident from lateral light sources into a restricted angular range in such a way that, in directions extending at angles α greater than γ, with γ>20°, to the surface normal of the screen, The light emanating from the screen is superimposed with the light deflected by the optical element, only from angles β<γ to the surface normal of the screen.
The methods and arrangements mentioned above have, as a rule, the common disadvantages that they distinctly reduce the brightness of the basic screen, and/or require an active, but at least a special, optical element for switching between the modes, and/or are complicated and expensive to fabricate, and/or degrade resolution in the free viewing mode.